1. Field of the Invention
The present invention relates to electromagnetic radiation detection systems. In particular, the present invention is directed to systems utilizing alternate layers of different semiconductor materials to provide a composite superlattice which is capable of being tuned such that the wavelengths of the peak response or energy band gap of the detector in response to electromagnetic radiation may be modulated.
2. Description of the Prior Art
Photo sensitive semiconductors are basically of two types, photoconductive and photovoltaic. When radiation of the proper energy falls upon a photoconductive semiconductor, the conductivity of the semiconductor increases. Energy supplied to the semiconductor causes covalent bonds to be broken, and electron-hole pairs in excess of those generated thermally are created. These increased current carriers decrease the resistance of the material. This "photoconductive effect" in semiconductor materials is sued in photoconductive detectors.
If, on the other hand, the semiconductor sensor is such that it incorporates a pn junction, it gives rise to electron-hole pairs which create a potential difference in response to radiation of the proper energy. This is referred to as a "photovoltaic" effect. While semiconductor electromagnetic radiation detectors can be of either form, the form which is characterized by the photoconductive effect is more generally used.
A photoconductive detector is normally a bar of semiconductor material having electrical contacts at the end. In its simplest form, the photoconductive detector is connected in series with a direct-current voltage source and a load resister. The change in resisitivity of the photoconductive detector in response to incident radiation is sensed in one of two ways. If the resistance of the load resister is much greater than the resistance of the detector, the device is operating in the "constant current mode", since the current through the detector is essentially constant. In this mode, the change in resistivity of the photoconductive detector is usually sensed by measuring the voltage across the photoconductive detector.
If, on the other hand, the resistance of the load resister is much less than the resistance of the detector, the photoconductive detector is operating in the "constant voltage mode", since the voltage across the photoconductive detector is essentially constant. The change in resistivity of the photoconductive detector is usually sensed by measuring the voltage across the load resister.
Of the two detector modes, the constant current mode finds wider use in photoconductive device detectors made from semiconductor materials having low resistivity. Most of the materials utilized for this photoconductive effect are materials of low resistivity and therefore the constant current mode is the normal method of measurement.
In the case of photovoltaic detectors, of course, any conventional method of measuring the voltage generated by the semiconductor may be employed. Usually, the constant current mode described above is used to measure the voltage generated across the photovoltaic detector. Photo detectors, and particularly photoconductive detectors, have found many applications in the prior art. One particularly important area is in the detector of infrared radiation or other types of radiation outside of the visible spectrum. In the prior art, photoconductive semiconductor materials of a given composition have given a peak electromagnetic radiation response over a very narrow band of wavelengths. Thus, the particular composition of the semiconductor material utilized was specifically designed for response in a narrow band. The only method by which the peak response band could be varied was by varying the composition of the semiconductor materials themselves.
For example, one widely used intrinsic infrared sensitive photodetector material is mercury cadmium telluride, which consists of a mixture of cadmium telluride and mercury telluride. Cadmium telluride is a wide gap semiconductor (Eg=1.6 eV), and mercury telluride is a semimetal having a "negative energy gap" of about -0.3 eV. In the composition of the mercury cadmium telluride alloy (designated Hg.sub.1-x Cd.sub.x Te), it has been found that the energy gap of the alloy varies linearly with x. By properly selecting x, it has previously been possible to obtain conductor materials having a peak response in any of a wide range of infrared wavelengths. Similarly, other composite photosensitive materials of both the photoconductive and photovoltaic type response by changing the composition to achieve a particular wavelength sensitivity.
More recently, it has been found that "superlattice" materials which are structures consisting of alternating layers of two different semiconductor materials, have a characteristic semimetal-to-semiconductor transition which can be influenced by the presence of a magnetic field. One such material is a superlattice consisting of alternating layers of indium arsenide and gallium antimonide (InAs-GaSb). The characteristic affect of a magnetic field on superlattices with two host semiconductors, particularly InAs and GaSb are discussed in "Kawai, N. J., et al" Magnetic Field-Induced Semimetal-to-Semiconductor Transition in InAs-GaSb Superlattices, Applied Physics Letters 36(5), 369 (March, 1980).
It can be seen from the above discussion, that highly sensitive photoconductive or photovoltaic sensors having a variation of peak response wavelengths can be produced. However, it has heretofore been necessary to change the composition of the detector material in order to vary the peak response wavelength. Therefore, for each peak wavelength or band gap required to be detected, a different material composition had to be used. Also, the number of substance combinations with proper material band gaps is limited and it may be difficult in some instances to produce a material with the appropriate band gap.